SDF was founded in Syria's mainly Kurdish northeastern region in October 2015 and is made up of at least 15 armed factions, mostly fighters from the Kurdish People's Protection Units and the Free Syrian Army.

Air strike casualties

The Observatory said in a report on Tuesday that at least 1,264 civilians, including 280 children, have been killed in US-led coalition air strikes since September 2014.

On April 14, the US Central Command said it mistakenly killed at least 18 members of the SDF in Tabqa.

Elsewhere in Syria, government air strikes have hit the town of Houla in Homs province, and killed at least six civilians, the Observatory reported.

Suspected Russian air strikes killed at least 15 people in Jabal al-Dweileh in Idlib province, the Syrian Civil Defence said on Tuesday.

In its latest report, the Observatory said at least 10,915 civilians, including 2,393 children, were killed in government air strikes and barrel bombs dropped by helicopters.

In addition to that, 64,000 others were injured in these attacks.

The casualties were a result of 30 months of air strikes and helicopter strikes.

The UN said in a press release on Monday that at least 39,000 newly displaced people fled to the Jib Al-Shaair makeshift camp in Raqqa province, where four out of five people are staying in the open air without appropriate shelter.

"The UN is deeply concerned for the safety and protection of over 400,000 people in Raqqa, in Syria. In past weeks, civilians have been exposed to daily fighting and air strikes which resulted in an escalating number of civilian deaths and injuries as well as damage to civilian infrastructure, including hospitals, schools, markets and water infrastructure," the press release said.

As the Syrian conflict enters its seventh year, more than 465,000 people have been killed in the fighting, more than one million injured and over 12 million Syrians - half the country's prewar population - have been displaced from their homes.